I shut down freeswitch, removed all the sofia_*.db files, and restarted freeswitch.
The error still occurs soon after log entries that say it pinged the providers (flowroute and sip2sip). The sofia_*.db files were recreated, and using sqlite3, they seem to have valid tables, although the sip_registrations table is empty in sofia_reg_external.db.
The symptom of this problem is that outgoing calls work, but incoming calls are rejected, and Flowroute redirects it to the backup (sip2sip voicemail).